One major challenge in the field of transplantation is the critical shortage of donor organs. While xenotransplantation might address this problem, its potential use is limited by the hyperacute rejection reaction that inevitably occurs when organs are transplanted between phylogenetically distant species. Hyperacute xenograft rejection is characterized by prominent evidence of vascular injury that is thought to be initiated by the reaction of natural antibodies and complement of the recipient with the endothelial lining of blood vessels of the donor organ. It is the overall objective of the proposed research to elucidate the pathogenesis of hyperacute xenograft rejection and to develop strategies to prevent it; the achievement of these goals will contribute to the application of xenotransplantation for amelioration of disease in humans. One specific aim of the proposed research is the further characterization of a porcine endothelial cell glycoprotein, gp135, which appears to be the most important target of human natural antibodies in the initiation of the rejection reaction. Monoclonal antibodies directed against this glycoprotein and/or partial amino acid sequences will be used to isolate and clone full length cDNA. The protein and cDNA sequences will be used to deduce the function of gp135 based on homology with other known sequences and as it relates to the pathophysiology of hyperacute rejection. To the extent that other porcine endothelial cell glycoproteins are found to be pathophysiologically important, these glycoproteins will be investigated as well. Another aim of this project is to define the roles of human natural antibodies and complement in mediating injury to xenogeneic endothelium. Human natural antibodies and human and murine monoclonal antibodies directed against specific porcine endothelial cell antigens, including gp135, will be tested with and without complement for the ability to stimulate or "activate" porcine endothelial cells. The manifestations of endothelial cell activation as it occurs will be related to the specific structures recognized by anti-endothelial cell antibodies and the active complement moieties or combination thereof required as a stimulus.